1. Field of the Invention
The present disclosure generally relates to the fabrication of semiconductor devices, and, more particularly, to methods for selectively forming a protective conductive cap on a metal gate electrode and devices resulting from the same.
2. Description of the Related Art
In modern integrated circuits, such as microprocessors, storage devices and the like, a large number of circuit elements, especially transistors, are provided on a restricted chip area. Immense progress has been made with respect to increased performance and reduced feature sizes of circuit elements; however, the ongoing demand for enhanced functionality of electronic devices forces semiconductor manufacturers to steadily reduce the dimensions of the circuit elements as well as increase their operating speed. Such continuous scaling of feature sizes involves great efforts in redesigning process techniques and developing new process strategies and tools.
For many device technology generations, the gate structures of most transistor elements have been based on the use of silicon and silicon-based materials, such as a silicon dioxide and/or silicon oxynitride gate dielectric layer, in combination with a polysilicon gate electrode. However, as the channel length of aggressively scaled transistor elements has become increasingly smaller, many newer generation devices have turned to gate structures that make use of alternative materials in an effort to avoid the short-channel effects which may be associated with the use of traditional silicon-based materials in reduced channel length transistors. For example, gate structures that utilize a so-called high-k dielectric/metal gate (“HK/MG”) configuration have been shown to provide significantly enhanced operational characteristics over the heretofore more commonly used silicon dioxide/polysilicon (“SiO/poly”) configurations.
Depending on the specific overall device requirements, several different high-k materials—i.e., materials having a dielectric constant, or k-value, of approximately 10 or greater—have been used with varying degrees of success for gate structures that utilize an HK/MG material layer stack. For example, in some transistor element designs, the high-k gate dielectric material may include tantalum oxide (Ta2O5), hafnium oxide (HfO2), zirconium oxide (ZrO2), titanium oxide (TiO2), aluminum oxide (Al2O3), hafnium silicates (HfSiOx) and the like. Furthermore, a metal material layer, often referred to as a work-function adjusting material layer, may be formed on the high-k gate dielectric material so as to control the work function of the transistor. These work-function adjusting materials may include, for example, titanium (Ti), titanium nitride (TiN), titanium-aluminum (TiAl), aluminum (Al), aluminum nitride (AlN), tantalum (Ta), tantalum nitride (TaN), tantalum carbide (TaC), tantalum carbonitride (TaCN), tantalum silicon nitride (TaSiN), tantalum silicide (TaSi) and the like. Additionally, a conductive metal gate electrode, e.g. aluminum, may be formed on the HK/MG material layer stack. The gate structure may be formed using the so-called “gate last” or “replacement gate” technique discussed below.
In the replacement gate technique, initial device processing steps are performed using a “dummy” gate structure. The term “dummy” gate structure refers to a process sequence wherein a gate structure that is formed during an early manufacturing stage does not ultimately form a part of the finished semiconductor device, but is instead removed and replaced with a replacement gate structure, e.g., a replacement metal gate structure, during a later manufacturing stage. However, a variety of problems can sometimes occur to a replacement metal gate structure during later device processing steps that can lead to device defects, and/or have a detrimental influence on overall device reliability. For example, when a contact structure is subsequently formed so as to provide an electrical connection to the replacement gate electrode, the material of the contact structure can sometimes enter and/or partially fill the gate cavity. In such cases, contact material could thereby have an effect on the threshold voltage (Vt) of the device. FIGS. 1A-1B illustrate a prior art process that can result in such a problem.
FIG. 1A illustrates a device 110 that includes a replacement metal gate structure 145 during a stage of forming contact elements to the device 110. The replacement metal gate structure 145 is formed over an active region 110A of the device 110 and includes a replacement gate electrode 140, such as an aluminum gate electrode, that is formed on and above an HK/MG material layer stack 130. Spacers 116 are formed on sidewalls of the replacement metal gate structure 145, and an interlayer dielectric material 117 covers the device 110 including the metal gate structure 145. The device 110 is formed above a semiconductor substrate 112 that includes source/drain regions 118. Isolation regions 113 are formed in the substrate 112 and define an active region 110A of the device 110. As shown in FIG. 1A, contact openings 180, 181 have been formed in the interlayer dielectric material 117 so as to expose the replacement gate electrode 140 and the source/drain regions 118, respectively.
FIG. 1B illustrates the device 110 after a conductive contact material, such as tungsten, has been formed in and above each of the openings 180, 181 and a planarization process has been performed to remove excess material from above the interlayer dielectric material 117, thereby forming a contact structure 200 in the opening 180 and contact structures 201 in the openings 181. As shown in FIG. 1B, a portion 200P of the contact structure 200 has entered and partially filled the replacement gate electrode 140. This situation sometimes occurs because the aluminum material of the replacement gate electrode 140 is vulnerable to attack during the etch/clean process that is used to form the opening 180 above the replacement gate electrode 140. Consequently, a portion of the aluminum material may be removed during the contact etch/clean process, and subsequently filled in by the material of the contact structure 200, e.g., tungsten. The presence of the material of the contact structure 200 within the replacement gate electrode 140 can cause an undesirable shift in the threshold voltage (Vt) of the device 110, thus degrading predictability and overall reliability of the device 110.
The present disclosure is directed to various processing methods and the resulting semiconductor devices that may be used to reduce the occurrence of, or even substantially eliminate, one or more of the problems described above.